Fix instances of gen_rtx_REG (VOIDmode, ...)

Fix instances of gen_rtx_REG (VOIDmode, ...)

Several definitions of INCOMING_RETURN_ADDR_RTX used
gen_rtx_REG (VOIDmode, ...), which with later patches
would trip an assert.  This patch converts them to use
Pmode instead.

gcc/
2016-11-15  Richard Sandiford  <richard.sandiford@arm.com>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

	* config/i386/i386.h (INCOMING_RETURN_ADDR_RTX): Use Pmode instead
	of VOIDmode.
	* config/ia64/ia64.h (INCOMING_RETURN_ADDR_RTX): Likewise.
	* config/iq2000/iq2000.h (INCOMING_RETURN_ADDR_RTX): Likewise.
	* config/m68k/m68k.h (INCOMING_RETURN_ADDR_RTX): Likewise.
	* config/microblaze/microblaze.h (INCOMING_RETURN_ADDR_RTX): Likewise.
	* config/mips/mips.h (INCOMING_RETURN_ADDR_RTX): Likewise.
	* config/mn10300/mn10300.h (INCOMING_RETURN_ADDR_RTX): Likewise.
	* config/nios2/nios2.h (INCOMING_RETURN_ADDR_RTX): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r242447